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.\" Copyright (c) 2013-2021 Enrico M. Crisostomo <enrico.m.crisostomo@gmail.com>
.\"
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.\"
.\"   Man page for the @FSWATCH@ command.
.\"
.\"   $Log$
.\"
.Dd @MAN_DATE@
.Dt "@FSWATCH@" "7" "@FSWATCH@ file system monitor"
.Os "@TARGET_VENDOR@" "@TARGET_OS@"
.Pp
.Sh NAME
.Nm @FSWATCH@
.Nd Ask for notification when the contents of the specified files or directory
hierarchies are modified.
.Pp
This man page is kept for reference but it is not to be considered an
authoritative or complete source of information.  Please, consult the official
Info documentation shipped with
.Nm .
.Sh SYNOPSIS
.Nm @FSWATCH@ (option)* path+
.Sh DESCRIPTION
The
.Nm
command receives notifications when the contents of the specified files or
directories are modified.  @FSWATCH@ implements six kind of monitors:
.Bl -tag -width indent
.It -
A monitor based on the File System Events API of Apple macOS.
.It -
A monitor based on kqueue, an event notification interface introduces in FreeBSD
4.1 and supported on most *BSD systems (including macOS).
.It -
A monitor based on the File Events Notification API of the Solaris kernel and
its derivatives.
.It -
A monitor based on inotify, a Linux kernel subsystem that reports file system
changes to applications.
.It -
A monitor based on the ReadDirectoryChangesW Microsoft Windows API.
.It -
A monitor which periodically stats the file system, saves file modification
times in memory and manually calculates changes.
.El
.Pp
.Nm
writes a record for each event it receives containing:
.Bl -tag -width indent
.It -
The timestamp when the event was
.Em received
(optionally).
.It -
The path affected by the current event.
.It -
A space-separated list of
.Em event types
(see
.Sx EVENT TYPES
).
.El
.Pp
.Nm
enters an infinite loop and never returns.  When it receives a SIGABRT, SIGINT
or SIGTERM signal,
.Nm
closes the notification streams and exits gracefully returning 0 to the calling
process.
.Pp
The following options are available:
.Bl -tag -width indent
.It Fl 0, -print0
Use the ASCII NUL character (\e0) as line separator.  Since file names can
potentially contain any character but NUL, this option assures that the output
of @FSWATCH@ can be safely parsed using NUL as delimiter, such as using xargs -0
and the shell builtin read -d ''.
.It Fl 1, -one-event
Exit fswatch after the first set of events is received.
.It Fl -batch-marker Ar marker
Print a marker at the end of every batch.  An optional marker
.Ar marker
can be specified to override its default value `NoOp'.
.It Fl -event Ar name
Filter event with the specified
.Ar name .
This option can be used multiple times, one for each event
.Ar name
that must be included in the output.
.It Fl e, -exclude Ar regexp
Exclude paths matching
.Ar regexp .
Multiple exclude filters can be specified using this option multiple times.  See
.Sx FILTERING PATHS
for further information.
.It Fl E, -extended
Use extended regular expressions.
.It Fl f, -format-time Ar format
Print the event time using the specified
.Ar format .
Supported formats are specified by
.Xr strftime 3 .
.It Fl h, -help
Show the help message.
.It Fl i, -include Ar regexp
Include paths matching
.Ar regexp .
Multiple include filters can be specified using this option multiple times.
See
.Sx FILTERING PATHS
for further information.
.It Fl I, -insensitive
Use case insensitive regular expressions.
.It Fl l, -latency Ar latency
Set the
.Ar latency
in seconds.
The latency is a double value greater than 0.1.
Smaller values are currently not allowed in order not to compromise the
performance of the system.
The default latency is 1 second.
.It Fl L, -follow-links
Follow symbolic links.
.It Fl M, -list-monitors
List the available monitors.
.It Fl m, -monitor Ar name
Uses the monitor specified by
.Ar name .
The list of currently available monitors can be obtained using the
.Fl h
option.
.It Fl n, -numeric
Print the numeric value of the event flag, instead of the array of symbolic
names.  The numeric value of the event flags are system-specific and may vary
across different versions of macOS.  As a consequence, the use of numeric values
is discouraged.
.It Fl o, -one-per-batch
Print a single message with the number of change events.
.It Fl r, -recursive
Watch subdirectories recursively.  This option may not be supported on all
systems.
.It Fl t, -timestamp
Print the event timestamp.
.It Fl u, -utf-time
Print the event time in UTC format.  When this option is not specified, the time
is printed using the system
.Em local
time, as defined by
.Sy localtime (3) .
.It Fl v, -verbose
Print verbose output.
.It Fl -version
Print the version of
.Nm
and exits.
.It Fl x, -event-flags
Prints the event flags.
.It Fl -event-flag-separator Ar separator
Print event flags using the specified
.Ar separator .
.El
.Sh MONITORS
.Nm
acts as a front-end to system-specific monitors.  Currently, the available
monitors are:
.Bl -tag -width indent
.It -
The
.Em FSEvents monitor ,
a monitor based on the File System Events API of Apple macOS.
.It -
The
.Em kqueue monitor ,
a monitor based on kqueue, an event notification interface introduced in FreeBSD
4.1 and supported on most *BSD systems (including macOS).
.It -
The
.Em inotify monitor ,
a Linux kernel subsystem that reports file system changes to applications.
.It -
The
.Em poll monitor ,
a monitor which periodically stats the file system, saves file modification
times in memory and manually calculates file system changes, which can work on
any operating system where
.Xr stat 2
can be used.
.El
.Pp
Each monitor has its own strengths, weakness and peculiarities.  Although
.Nm
strives to provide a uniform experience no matter which monitor is used, it is
still important for users to know which monitor they are using and to be aware
of existing bugs, limitations, corner cases or pathological behaviour.
.Ss The FSEvents Monitor
The
.Em FSEvents monitor ,
available only on Apple macOS, has no known limitations and scales very well with
the number of files being observed.  In fact, I observed no performance
degradation when testing
.Nm
observing changes on a filesystem of 500 GB over long periods of time.  On macOS,
this is the default monitor.
.Ss The kqueue Monitor
The
.Em kqueue monitor ,
available on any *BSD system featuring kqueue, requires a file descriptor to be
opened for every file being watched.  As a result, this monitor scales badly
with the number of files being observed and may begin to misbehave as soon as
the
.Nm
process runs out of file descriptors.  In this case,
.Nm
dumps one error on standard error for every file that cannot be opened.  Beware
that on some systems the maximum number of file descriptors that can be opened
by a process is set to a very low value (values as low as 256 are not uncommon),
even if the operating system may allow a much larger value.
.Pp
If you are running out of file descriptors when using this monitor and you
cannot reduce the number of observed items, either:
.Bl -tag -width indent
.It -
Consider raising the number of maximum open file descriptors (check your OS
documentation).
.It -
Consider using another monitor.
.El
.Ss The inotify Monitor
The
.Em inotify monitor ,
available on Linux since kernel 2.6.13, may suffer a queue overflow if events
are generated faster than they are read from the queue.  In any case, the
application is guaranteed to receive an overflow notification which can be
handled to gracefully recover.
.Nm
currently throws an exception if a queue overflow occurs.  Future versions will
handle the overflow by emitting proper notifications.  However, the odds of
observing a queue overflow on a default configured mainstream GNU/Linux
distribution is very low.
.Pp
The inotify API sends events for the direct child elements of a watched
directory and it scales pretty well with the number of watched items.  For this
reason, depending on the number of files to watch, it may sometimes be
preferable to watch a common parent directory and filter received events rather
than adding a huge number of file watches.
.Ss The Poll Monitor
The
.Em poll monitor
was added as a fallback mechanisms in the cases where no other monitor could be
used, including:
.Bl -tag -width indent
.It -
Operating system without any sort of file events API.
.It -
Situations where the limitations of the available monitors cannot be overcome
(i.e.: observing a number of files greater than the available file descriptors
on a system using the kqueue monitor).
.El
.Pp
The poll monitor, available on any platform, only relies on available CPU and
memory to perform its task (besides the
.Xr stat 2
function).  The performance of this monitor degrades linearly with the number of
files being watched.  The authors' experience indicates that
.Nm
requires approximately 150 MB or RAM memory to observe a hierarchy of 500.000
files with a minimum path length of 32 characters.  A common bottleneck of the
poll monitor is disk access, since stat()-ing a great number of files may take a
huge amount of time.  In this case, the latency should be set to a sufficiently
large value in order to reduce the performance degradation that may result from
frequent disk access.
.Ss How to Choose a Monitor
.Nm
already chooses the "best" monitor for your platform if you do not specify any.
However, a specific monitor may be better suited to specific use cases.  Please,
read the
.Sx MONITORS
section to get a description of all the available monitors and their
limitations.
.Pp
Usage recommendations are as follows:
.Bl -tag -width indent
.It -
On macOS, use only the FSEvents monitor (which is the default behaviour).
.It -
On Linux, use the inotify monitor (which is the default behaviour).
.It -
If the number of files to observe is sufficiently small, use the kqueue monitor.
Beware that on some systems the maximum number of file descriptors that can be
opened by a process is set to a very low value (values as low as 256 are not
uncommon), even if the operating system may allow a much larger value.  In this
case, check your OS documentation to raise this limit on either a per process or
a system-wide basis.
.It -
If feasible, watch directories instead of watching files.  Properly crafting the
receiving side of the events to deal with directories may sensibly reduce the
monitor resource consumption.
.It -
If none of the above applies, use the poll monitor.  The authors' experience
indicates that fswatch requires approximately 150 MB or RAM memory to observe a
hierarchy of 500.000 files with a minimum path length of 32 characters.  A
common bottleneck of the poll monitor is disk access, since stat()-ing a great
number of files may take a huge amount of time.  In this case, the latency
should be set to a sufficiently large value in order to reduce the performance
degradation that may result from frequent disk access.
.El
.Sh FILTERING PATHS
Received events can be filtered by path using regular expressions.  Regular
expressions can be used to include or exclude matching paths.  The user can
specify multiple filter expression in any order and the
.Em first
matching expression wins.
.Pp
Other options govern how regular expressions are interpreted:
.Bl -tag -width indent
.It -
Regular expressions can be
.Em extended
if option
.Fl E
is specified.
.It -
Regular expressions can be
.Em case insensitive
if option
.Fl I
is specified.
.El
.Sh EVENT TYPES
Event flags identify the kind of change a file system object has undergone. Many of them directly map to common file system operations (such as creation, deletion, update, etc.), others are less common (such as attribute modification), and others are monitor and platform specific.
.sp
Currently,
.Nm
maps monitor-specific event flags to 'global' event flags acting as a sort of 'greatest common denominator' of all the available monitor flags. The following global event flags are available::
.Bl -tag -width 20n
.It Sy NoOp
Idle event, optionally issued when no changes were detected.
.It Sy PlatformSpecific
This event maps a platform-specific event that has no corresponding flag.
.It Sy Created
The object has been created.
.It Sy Updated
The object has been updated. The kind of update is monitor-dependent.
.It Sy Removed
The object has been removed.
.It Sy Renamed
The object has been renamed.
.It Sy OwnerModified
The object’s owner has changed.
.It Sy AttributeModified
An object’s attribute has changed.
.It Sy MovedFrom
The object has moved from this location to a new location of the same file system.
.It Sy MovedTo
The object has moved from another location in the same file system into this location.
.It Sy IsFile
The object is a regular file.
.It Sy IsDir
The object is a directory.
.It Sy IsSymLink
The object is a symbolic link.
.It Sy Link
The object link count has changed.
.It Sy Overflow
The monitor has overflowed.
.El
.Sh EXAMPLES
.Ss Basic Usage
.Nm
syntax is the following:
.Pp
.Dl $ fswatch [options] [paths] ...
.Pp
.Nm
will then output change events to standard output. By default, only the affected
file name is printed.  However, many options are available to format the event
record, including:
.Bl -tag -width indent
.It -
The possibility of adding the event timestamp.
.It -
The possibility of adding the event mask in both textual and numerical form.
.El
.Pp
The following command listens for changes in the current directory and events
are delivered every 5 seconds:
.Pp
.Dl "$ @FSWATCH@ -l 5 ."
.Pp
The following command listens for changes in the current user home directory and
.Em /var/log :
.Pp
.Dl "$ @FSWATCH@ ~ /var/log"
.Ss Piping @FSWATCH@ Output to Another Process
Very often you wish to not only receive an event, but react to it.  The simplest
way to do it is piping fswatch output to another process.  Since in Unix and
Unix-like file system file names may potentially contain any character but
.Em NUL (\e0)
and the path separator
.Em (/) ,
.Nm
has a specific mode of operation when its output must be piped to another
process.  When the
.Op Fl 0
option is used,
.Nm
will use the
.Em NUL
character as record separator, thus allowing any other character to appear in a
path.  This is important because many commands and shell builtins (such as
.Em read )
split words and lines by default using the characters in
.Em $IFS ,
which by default contains characters which may be present (although rarely) in a
file name, resulting in a wrong event path being received and processed.
.Pp
Probably the simplest way to pipe
.Nm
to another program in order to respond to an event is using
.Em xargs :
.Pp
.Dl "$ fswatch -0 [opts] [paths] | xargs -0 -n 1 -I {} [command]"
.Pp In this example:
.Bl -tag -width indent
.It -
.Em fswatch -0
will split records using the
.Em NUL
character.
.It -
.Em xargs -0
will split records using the
.Em NUL
character. This is required to correctly match impedance with
.Nm .
.It -
.Em xargs -n 1
will invoke
.Em command
every record.
If you want to do it every
.Em x
records, then use
.Em xargs -n x .
.It -
.Em xargs -I {}
will substitute occurrences of
.Em {}
in command with the parsed argument.  If the command you are running does not
need the event path name, just delete this option.  If you prefer using another
replacement string, substitute
.Em {}
with yours.
.El
.Ss Bubbling Events
An often requested feature is being able to receive a single event "per batch",
instead of receiving multiple events.  This use case is implemented by the
.Op Fl o, -one-per-batch
option which tells
.Nm
to dump a record containing the number of received events, without any other
detail:
.Pp
.Dl $ fswatch -or /path/to/watch
.Dl 1
.Dl 10
.Dl [...]
.Pp
This is useful if, for example, you want to respond to change events in a way
which is (or can easily be) path-independent (because you are not receiving any
event detail) and you prefer to "bubble" events together to reduce the overhead
of the command being executed.  A typical case is a directory synchronisation
job whenever some files change.
.Ss Receiving a Single Event
Another requested feature is the possibility of receiving a single event and
exit.  This is most useful when existing scripts processing events include the
restart logic of
.Nm
This use case is implemented by the
.Op Fl 1, -one-event
option:
.Pp
.Dl $ fswatch -1 /path/to/watch
.Dl /path/to/watch
.Sh Compatibility With @FSWATCH@ 0.x
The previous major version of
.Nm
(v. 0.x) allowed users to run a command whenever a set of changes was detected
with the following syntax:
.Pp
.Dl $ fswatch path program
.Pp
Starting with
.Nm
v. 1.x this behaviour is no longer supported.  The rationale behind this
decision includes:
.Bl -tag -width indent
.It -
The old version only allows watching one path.
.It -
The command to execute was passed as last argument, alongside the path to watch,
making it difficult to extend the program functionality to add multiple path
support
.It -
The old version forks and executes /bin/bash, which is neither portable, nor
guaranteed to succeed, nor desirable by users of other shells.
.It -
No information about the change events is passed to the forked process.
.El
.Pp
To solve the aforementioned issues and keep
.Nm
consistent with common UNIX practices, the behaviour has changed and
.Nm
now prints event records to the standard output that users can process further
by piping the output of
.Nm
to other programs.
.Pp
To fully support the old use, the
.Op Fl o, -one-per-batch
option was added in v. 1.3.3.
When specified,
.Nm
will only dump 1 event to standard output which can be used to trigger another
program:
.Pp
.Dl $ fswatch -o path | xargs -n1 program
.Pp
In this case, program will receive the number of change events as first
argument.  If no argument should be passed to program, then the following
command could be used:
.Pp
.Dl $ fswatch -o path | xargs -n1 -I{} program
.Sh EXIT STATUS
.Nm
may exit with one of the following exit statuses:
.Pp
.Dl FSW_OK                            0
.Dl FSW_ERR_UNKNOWN_ERROR             (1 << 0)
.Dl FSW_ERR_SESSION_UNKNOWN           (1 << 1)
.Dl FSW_ERR_MONITOR_ALREADY_EXISTS    (1 << 2)
.Dl FSW_ERR_MEMORY                    (1 << 3)
.Dl FSW_ERR_UNKNOWN_MONITOR_TYPE      (1 << 4)
.Dl FSW_ERR_CALLBACK_NOT_SET          (1 << 5)
.Dl FSW_ERR_PATHS_NOT_SET             (1 << 6)
.Dl FSW_ERR_UNKNOWN_MONITOR           (1 << 7)
.Dl FSW_ERR_MISSING_CONTEXT           (1 << 8)
.Dl FSW_ERR_INVALID_PATH              (1 << 9)
.Dl FSW_ERR_INVALID_CALLBACK          (1 << 10)
.Dl FSW_ERR_INVALID_LATENCY           (1 << 11)
.Dl FSW_ERR_INVALID_REGEX             (1 << 12)
.Dl FSW_ERR_MONITOR_ALREADY_RUNNING   (1 << 13)
.Dl FSW_ERR_STALE_MONITOR_THREAD      (1 << 14)
.Dl FSW_ERR_THREAD_FAULT              (1 << 15)
.Dl FSW_ERR_UNSUPPORTED_OPERATION     (1 << 16)
.Dl FSW_ERR_UNKNOWN_VALUE             (1 << 17)
.Sh DIAGNOSTICS
.Nm
exits 0 on success, and >0 if an error occurs.
.Sh COMPATIBILITY
.Nm
can be built on any system supporting at least one of the available monitors.
.Sh BUGS
See https://github.com/emcrisostomo/fswatch/issues
for open issues or to create a new one.
.Pp
Bugs can also be submitted to @MAN_BUG_REPORT@.
